The next generation mobile wireless communication system (5G or NR), will support a diverse set of use cases and a diverse set of deployment scenarios. The latter includes deployment at both low frequencies (100 s of MHz), similar to LTE today, and very high frequencies (mm waves in the tens of GHz). At high frequencies, propagation characteristics make achieving good coverage challenging. One solution to the coverage issue is to employ high-gain beamforming, typically in an analog manner, in order to achieve satisfactory link budget. Beamforming will also be used at lower frequencies (typically digital beamforming), and is expected to be similar in nature to the already standardized 3GPP LTE system (4G).
For background purposes, some of the key aspects of LTE are described in this section. Of particular relevance is the sub-section describing channel state information reference signals (CSI-RS). A similar signal will be designed also for NR, and is the subject of the present application.
Note that terminology used here such as eNodeB and UE should be considering non-limiting and does in particular not imply a certain hierarchical relation between the two; in general, “eNodeB” could be considered as device 1 and “UE” device 2, and these two devices communicate with each other over some radio channel. Herein, we also focus on wireless transmissions in the downlink, but the invention is equally applicable in the uplink.
LTE and NR use OFDM in the downlink and DFT-spread OFDM or OFDM in the uplink. The basic LTE or NR downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 6, where each resource element corresponds to one OFDM subcarrier during one OFDM symbol interval.
Moreover, as shown in FIG. 7, in the time domain, LTE downlink transmissions are organized into radio frames of 10 milliseconds, each radio frame consisting of ten equally-sized subframes of length Tsubframe=1 millisecond.
Furthermore, the resource allocation in LTE is typically described in terms of resource blocks, where a resource block corresponds to one slot (0.5 millisecond) in the time domain and 12 contiguous subcarriers in the frequency domain. Resource blocks are numbered in the frequency domain, starting with 0 from one end of the system bandwidth. For NR, a resource block is also 12 subcarriers in frequency, but the number of OFDM symbols in the NR resource block has not yet been determined. It will be appreciated that the term “resource block,” as used herein, will thus refer to a block of resources spanning a certain number of subcarriers and a certain number of OFDM symbols—the term as used herein may, in some instances, refer to a different sized block of resources from what is ultimately labeled a “resource block” in the standards for NR or in the standards for some other system.
Downlink transmissions are dynamically scheduled, i.e., in each subframe the base station transmits control information about to which terminals data is transmitted and upon which resource blocks the data is transmitted, in the current downlink subframe. This control signaling is typically transmitted in the first 1, 2, 3 or 4 OFDM symbols in each subframe in LTE, and in 1 or 2 OFDM symbols in NR. A downlink system with 3 OFDM symbols as control is illustrated in the downlink subframe illustrated in FIG. 8.
Codebook-Based Precoding
Multi-antenna techniques can significantly increase the data rates and reliability of a wireless communication system. The performance is particularly improved if both the transmitter and the receiver are equipped with multiple antennas, which results in a multiple-input multiple-output (MIMO) communication channel. Such systems and/or related techniques are commonly referred to as MIMO.
NR is currently evolving with MIMO support. A core component in NR is the support of MIMO antenna deployments and MIMO related techniques including beamforming at higher carrier frequencies. Currently, LTE and NR support an 8-layer spatial multiplexing mode for up to 32 Tx antennas with channel-dependent precoding. The spatial multiplexing mode is aimed for high data rates in favorable channel conditions. An illustration of the spatial multiplexing operation is provided in FIG. 9.
As seen, the information carrying symbol vector s is multiplied by an NT×r precoder matrix W, which serves to distribute the transmit energy in a subspace of the NT—(corresponding to NT antenna ports) dimensional vector space. The precoder matrix is typically selected from a codebook of possible precoder matrices, and typically indicated by means of a precoder matrix indicator (PMI), which specifies a unique precoder matrix in the codebook for a given number of symbol streams. The r symbols in s each correspond to a layer and r is referred to as the transmission rank. In this way, spatial multiplexing is achieved, since multiple symbols can be transmitted simultaneously over the same time/frequency resource element (TFRE). The number of symbols r is typically adapted to suit the current channel properties.
LTE and NR use OFDM in the downlink and hence the received NR×1 vector yn for a certain TFRE on subcarrier n (or alternatively data TFRE number n) is thus modeled byyn=HnWsn+en where en is a noise/interference vector obtained as realizations of a random process. The precoder, implemented by precoder matrix W, can be a wideband precoder that is constant over frequency or that is frequency selective.
The precoder matrix is often chosen to match the characteristics of the NR×NT MIMO channel matrix Hn, resulting in so-called channel-dependent precoding. This is also commonly referred to as closed-loop precoding and essentially strives for focusing the transmit energy into a subspace which is strong in the sense of conveying much of the transmitted energy to the UE. In addition, the precoder matrix may also be selected to strive for orthogonalizing the channel, meaning that after proper linear equalization at the UE, the inter-layer interference is reduced.
The transmission rank, and thus the number of spatially multiplexed layers, is reflected in the number of columns of the precoder. For efficient performance, it is important that a transmission rank that matches the channel properties is selected.
Channel State Information Reference Symbols (CSI-RS)
In LTE and NR, a reference symbol sequence was introduced for the purpose of estimating channel-state information, the CSI-RS. The CSI-RS provides several advantages over basing the CSI feedback on the common reference symbols (CRS) which were used, for that purpose, in previous releases. Firstly, the CSI-RS is not used for demodulation of the data signal, and thus does not require the same density (i.e., the overhead of the CSI-RS is substantially less). Secondly, CSI-RS provides a much more flexible means to configure CSI feedback measurements (e.g., which CSI-RS resource to measure on can be configured in a UE specific manner).
By measuring on a CSI-RS, a UE can estimate the effective channel the CSI-RS is traversing, including the radio propagation channel and antenna gains. In more mathematical rigor, this implies that if a known CSI-RS signal x is transmitted, a UE can estimate the coupling between the transmitted signal and the received signal (i.e., the effective channel). Hence if no virtualization is performed in the transmission, the received signal y can be expressed asy=Hx+e and the UE can estimate the effective channel H.
Up to 32 CSI-RS ports can be configured for a LTE or NR UE, that is, the UE can thus estimate the channel from up to eight transmit antennas.
An antenna port is equivalent to a reference signal resource that the UE shall use to measure the channel. Hence, a base station with two antennas could define two CSI-RS ports, where each port is a set of resource elements in the time frequency grid within a subframe or slot. The base station transmits each of these two reference signals from each of the two antennas so that the UE can measure the two radio channels and report channel state information back to the base station based on these measurements. In LTE, CSI-RS resources with 1, 2, 4, 8, 12, 16, 20, 24, 28 and 32 ports are supported.
The CSI-RS utilizes an orthogonal cover code (OCC) of length two, to overlay two antenna ports on two consecutive REs. As seen in FIG. 10, which depicts resource element grids over an RB pair with potential positions for LTE Rel-9/10 UE specific RS (yellow), CSI-RS (marked with a number corresponding to the CSI-RS antenna port), and CRS (blue and dark blue), many different CSI-RS patterns are available. For the case of 2 CSI-RS antenna ports there are 20 different patterns within a subframe. The corresponding number of patterns is 10 and 5 for 4 and 8 CSI-RS antenna ports, respectively. For TDD, some additional CSI-RS patterns are available.
The CSI reference signal configurations are given by the table below, taken from LTE specifications TS 36.211 v.12.5.0. For example, the CSI RS configuration 5 for 4 antennas ports use (k′,l′)=(9,5) in slot 1 (the second slot of the subframe), and according to the formulas below, port 15,16, use OCC over the resource elements (k,l)=(9,5), (9,6) and port 17,18 use OCC over resource elements (3,5)(3,6) respectively (assuming PRB index m=0), where k is the subcarrier index and l is the OFDM symbol index.
The orthogonal cover code (OCC) is introduced below by the factor wl″
          ⁢      k    =                  k        ′            +              12        ⁢                                  ⁢        m            +              {                                                                                                  -                    0                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  15                          ,                          16                                                }                                                              ,                                          normal                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    6                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  17                          ,                          18                                                }                                                              ,                                          normal                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    1                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  19                          ,                          20                                                }                                                              ,                                          normal                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    7                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  21                          ,                          22                                                }                                                              ,                                          normal                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    0                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  15                          ,                          16                                                }                                                              ,                                          extended                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    3                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  17                          ,                          18                                                }                                                              ,                                          extended                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    6                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  19                          ,                          20                                                }                                                              ,                                          extended                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                                                                        -                    9                                                                                                                                      for                        ⁢                                                                                                  ⁢                        p                                            ∈                                              {                                                  21                          ,                          22                                                }                                                              ,                                          extended                      ⁢                                                                                          ⁢                      cyclic                      ⁢                                                                                          ⁢                      prefix                                                                                            ⁢                                                  ⁢            l                    =                                    l              ′                        +                          {                                                                                                                                            l                          ″                                                                                                                                                  CSI                            ⁢                                                                                                                  ⁢                            reference                            ⁢                                                                                                                  ⁢                            signal                            ⁢                                                                                                                  ⁢                            configurations                            ⁢                                                                                                                  ⁢                            0                            ⁢                                                          -                                                        ⁢                            19                                                    ,                                                      normal                            ⁢                                                                                                                  ⁢                            cyclic                            ⁢                                                                                                                  ⁢                            prefix                                                                                                                                                                                        2                          ⁢                                                                                                          ⁢                                                      l                            ″                                                                                                                                                                            CSI                            ⁢                                                                                                                  ⁢                            reference                            ⁢                                                                                                                  ⁢                            signal                            ⁢                                                                                                                  ⁢                            configurations                            ⁢                                                                                                                  ⁢                            20                            ⁢                                                          -                                                        ⁢                            31                                                    ,                                                      normal                            ⁢                                                                                                                  ⁢                            cyclic                            ⁢                                                                                                                  ⁢                            prefix                                                                                                                                                                                        l                          ″                                                                                                                                                  CSI                            ⁢                                                                                                                  ⁢                            reference                            ⁢                                                                                                                  ⁢                            signal                            ⁢                                                                                                                  ⁢                            configurations                            ⁢                                                                                                                  ⁢                            0                            ⁢                                                          -                                                        ⁢                            27                                                    ,                                                      extended                            ⁢                                                                                                                  ⁢                            cyclic                            ⁢                                                                                                                  ⁢                            prefix                                                                                                                                ⁢                                                                          ⁢                                                                          ⁢                                      w                                          l                      ″                                                                      =                                  {                                                                                                                                                                        1                                                                                                                      p                                ∈                                                                  {                                                                      15                                    ,                                    17                                    ,                                    19                                    ,                                    21                                                                    }                                                                                                                                                                                                                                                                          (                                                                      -                                    1                                                                    )                                                                                                  l                                  ″                                                                                                                                                                                    p                                ∈                                                                  {                                                                      16                                    ,                                    18                                    ,                                    20                                    ,                                    22                                                                    }                                                                                                                                                                    ⁢                                                                                                  ⁢                                                                                                  ⁢                                                  l                          ″                                                                    =                      0                                        ,                                                                  1                        ⁢                                                                                                  ⁢                                                                                                  ⁢                        m                                            =                      0                                        ,                    1                    ,                    …                    ⁢                                                                                  ,                                                                                            N                          RB                          DL                                                -                                                  1                          ⁢                                                                                                          ⁢                                                                                                          ⁢                                                      m                            ′                                                                                              =                                              m                        +                                                  ⌊                                                                                                                    N                                RB                                                                  max                                  ,                                  DL                                                                                            -                                                              N                                RB                                DL                                                                                      2                                                    ⌋                                                                                                                                                            
TABLE 6.10.5.2-1Mapping from CSI reference signal configuration to (k′, l′) for normal cyclic prefixCSI referenceNumber of CSI reference signals configuredsignal1 or 248configuration(k′, l′)ns mod 2(k′, l′)ns mod 2(k′, l′)ns mod 2Frame0(9, 5)0(9, 5)0(9, 5)0structure1(11, 2) 1(11, 2) 1(11, 2) 1type 12(9, 2)1(9, 2)1(9, 2)1and 23(7, 2)1(7, 2)1(7, 2)14(9, 5)1(9, 5)1(9, 5)15(8, 5)0(8, 5)06(10, 2) 1(10, 2) 17(8, 2)1(8, 2)18(6, 2)1(6, 2)19(8, 5)1(8, 5)110(3, 5)011(2, 5)012(5, 2)113(4, 2)114(3, 2)115(2, 2)116(1, 2)117(0, 2)118(3, 5)119(2, 5)1Frame20(11, 1) 1(11, 1) 1(11, 1) 1structure21(9, 1)1(9, 1)1(9, 1)1type 222(7, 1)1(7, 1)1(7, 1)1only23(10, 1) 1(10, 1) 124(8, 1)1(8, 1)125(6, 1)1(6, 1)126(5, 1)127(4, 1)128(3, 1)129(2, 1)130(1, 1)131(0, 1)12D Antenna Arrays
In LTE, support for two-dimensional antenna arrays was introduced where each antenna element has an independent phase and amplitude control, thereby enabling beamforming in both in the vertical and the horizontal dimensions. Such antenna arrays may be (partly) described by the number of antenna columns corresponding to the horizontal dimension Nh, the number of antenna rows corresponding to the vertical dimension Nv, and the number of dimensions corresponding to different polarizations Np. The total number of antennas is thus N=NhNvNp. An example of an antenna where Nh=8 and Nv=4 is illustrated in FIG. 11, which illustrates on the left side thereof a two-dimensional antenna array of cross-polarized antenna elements (Np=2), with Nh=4 horizontal antenna elements and Nv=8 vertical antenna elements, and on the right side of FIG. 11 the actual port layout with 2 vertical ports and 4 horizontal ports is illustrated. This could for instance be obtained by virtualizing each port by 4 vertical antenna elements. Hence, assuming cross-polarized ports are present, the UE will measure 16 antenna ports in this example.
However, from a standardization perspective, the actual number of elements antenna array is not visible to the UE, but rather the antenna ports, where each ports corresponds to a CSI reference signal. The UE can thus measure the channel from each of these ports. Therefore, we introduce a 2D port layout, described by the number of antenna ports in the horizontal dimension Mh, the number of antenna rows corresponding to the vertical dimension Mv and the number of dimensions corresponding to different polarizations Mp. The total number of antenna ports is thus M=MhMvMp. The mapping of these ports on to the N antenna elements is an eNB implementation issue and thus not visible by the UE. The UE does not even know the value of N; it only knows the value of the number of ports M.
Precoding may be interpreted as multiplying the signal with different beamforming weights for each antenna port prior to transmission. A typical approach is to tailor the precoder to the antenna form factor, i.e. taking into account Mh, Mv and Mp when designing the precoder codebook.
A common approach when designing precoder codebooks tailored for 2D antenna arrays is to combine precoders tailored for a horizontal array and a vertical array of antenna ports respectively by means of a Kronecker product. This means that (at least part of) the precoder can be described as a function ofWH⊗WV 
where WH is a horizontal precoder taken from a (sub)-codebook XH containing NH codewords and similarly WV is a vertical precoder taken from a (sub)-codebook XV containing NV codewords. The joint codebook, denoted by XH⊗XV, thus contains NH·NV codewords. The codewords of XH are indexed with k=0, . . . , NH−1, the codewords of XV are indexed with l=0, . . . , NV−1 and the codewords of the joint codebook XH⊗XV are indexed with m=NV·k+l meaning that m=0, . . . , NH·NV−1.
For LTE Rel-12 UE and earlier, only a codebook feedback for a 1D port layout is supported, with 2,4 or 8 antenna ports. Hence, the codebook is designed assuming these ports are arranged on a straight line.
Periodic CSI Reporting on a Subset of 2D Antenna Ports
A method has been proposed to use measurements on fewer CSI-RS ports for periodic CSI reports than measurements for the aperiodic CSI reports.
In one scenario, the periodic CSI report framework is identical to legacy terminal periodic CSI report framework. Hence, periodic CSI reports with 2, 4 or 8 CSI-RS ports are used for the P-CSI reporting and additional ports are used for the A-CSI reporting. From UE and eNB perspective, the operations related to periodic CSI reporting is identical to legacy operation.
The full, large 2D port layout CSI measurements of up to 64 ports or even more is only present in the aperiodic reports. Since A-CSI is carried over PUSCH, the payload can be much larger than the small 11-bit limit of the P-CSI using PUCCH format 2.
CSI-RS resource allocation for a 2D antenna array
It has been agreed that for 12 or 16 ports, a CSI-RS resource for class A CSI reporting is composed as an aggregation of K CSI-RS configurations each with N ports. In case of CDM-2, the K CSI-RS resource configurations indicate CSI-RS RE locations according to legacy resource configurations in TS36.211. For 16 ports:(N,K)=(8,2),(2,8)
For 12 port construction:(N,K)=(4,3),(2,6)
The ports of the aggregated resource correspond to the ports of component resources according to the following:
The aggregated port numbers are 15, 16, . . . 30 (for 16 CSI-RS ports)
The aggregated port numbers are 15, 16, . . . 26 (for 12 CSI-RS ports)
CSI-RS Antenna Port Numbering
For a given P antenna ports, the Rel-10,12 and 13 precoding codebooks are designed so that the P/2 first antenna ports (e.g. 15-22) should map to a set of co-polarized antennas and the P/2 last antenna ports (e.g. 16-30) are mapped to another set of co-polarized antennas, with an orthogonal polarization to the first set. This is thus targeting cross-polarized antenna arrays. FIG. 12 illustrates antenna port numbering for a case of P=8 ports.
Hence, the codebook principles for the rank 1 case are that a DFT “beam” vector is chosen for each set of P/2 ports and a phase shift with QPSK alphabet is used to co-phase the two sets of antenna ports. A rank 1 codebook is thus constructed as
         (                            a                                                  ae                          i              ⁢                                                          ⁢              ω                                            )  
where a is a length P/2 vector that forms a beam for the first and second polarizations respectively and ω is a co-phasing scalar that co-phases the two orthogonal polarizations.
Using CSI-RS Signals in NR
In NR, the CSI-RS signal needs to be designed and used for at least similar purposes as in LTE. However, the NR CSI-RS is expected to fulfill additional purposes such as beam management. Beam management is a process whereby eNB and UE beams are tracked which includes finding, maintaining, and switching between suitable beams as UEs move both within and between the coverage areas of multi-beam transmit-receive points (TRPs). This is accomplished by UEs performing measurements on the CSI-RS reference signals and feeding these measurements back to the network for the purposes of beam management decisions.
It is thus a problem how to design a CSI-RS that can be used for “LTE type” of functionality as well as for beam management functionality with both digital and analog beamforming.
An additional point of difference between NR and LTE is that NR will support flexible numerology, i.e., scalable sub-carrier spacing (SCS) with a nominal value of 15 kHZ. The nominal value is scalable in powers of 2, i.e., fSC=15*2n kHz where n=−2, −1, 0, 1, 2, 3, 4, 5. This affects the CSI-RS structure, as larger subcarrier spacings mean that resource elements (REs) can become more spread out in the frequency dimension and this results in a larger distance in frequency between CSI-RS. It is thus a problem how to design CSI-RS to be able to adjust the frequency density depending on the SCS.
One more possible point of difference is that NR may support a shorter transmission duration than LTE. The NR transmission duration is a slot where a slot can be either 7 or 14 OFDM symbols long. In contrast, the transmission duration in LTE is fixed at one subframe which equals 14 symbols.
Additionally, because there is no common reference signals (CRS) in NR, the placement of CSI-RS in NR is not restricted to avoid collisions with NR. Thus, greater flexibility may be used in the design of CSI-RS for NR.